1. Field of the Invention
The present invention relates to a dental alginate impression materials provided in powdery form and, more particularly, to a low-dusting powdery alginate impression material for dental purposes, which is characterized by reducing scattering of dust and excelling in storability and the accuracy of gypsum models.
2. Prior Art
Dental alginate impression materials provided in powdery form have been frequently used over a long term of years, because they are inexpensive and can be employed to prepare an impression in the mouth of suitable accuracy. In use, a predetermined amount of a dental powdery alginate impression material (hereinafter referred to as the alginate impression material) is placed with water in a small rubber bowl, in which they are mixed together with the use of a spatula into a paste. Thereafter, the paste is placed on an impression tray and inserted under pressure into the mouth. After the paste has gelled into an elastomer, it is removed to take an impression in the mouth. Then, gypsum slurry is poured into a negative cast of such an impression to obtain a gypsum model which is usable as a working model for prostheses.
How the surface details of gypsum is reproduced depends upon the fitness of the prepared prosthesis when it is retained in the oral mouth. The surface roughness of the gypsum model is governed by the boundary correlation between the alginate impression material and a gypsum model material. To this end, a material, which promotes or does not interfering with the gelling reaction of the alginate impression material and the solidification of gypsum, should practically be selected and used.
The alginate impression material comprises finely powdered components in order that it is easily formed with water into a paste of which a homogeneous gel elastomer is then formed. In particular, 50 to 80% by weight of finely divided particle powders such as diatomaceous earth, silicic anhydride, talc, calcium carbonate and perlite are generally used for the purpose of reinforcing the gel elastomer.
Powders of such an alginate impression material have a property of settling down during storage and changing gradually in their own bulk density. For that reason, a storage vessel is ordinarily restored to the lowest bulk density by shaking or other means to accurately take a predetermined amount of such powders therefrom with the use of an exclusive spoon having a constant volume.
As the storage vessel is uncovered after shaking, the powders tend to scatter into the air in the form of dust. Dusting also takes place by agitation made with a spatula when a predetermined amount of the powders is mixed with water into a paste in a small bowl of rubber. As heretofore pointed out, a grave problem with the alginate impression material is dusting, since dust does not only give an unpleasant feeling to users but also poses air pollution and health problems.
In order to solve such problems, it has been proposed to coat powder particles of the alginate impression material with a coating agent readily soluble and rapidly wettable in water, as disclosed in Japanese Patent Laid-Open No. 57-501426.
According to the teachings of the specification of the above published patent, the coating agents to be used include natural polymer dispersants such as xanthane rubber and sodium polyalginate; cellulose esters or ethers such as hydroxyethyl cellulose and carboxymethyl cellulose; synthetic nonionic polymer surfactants derived from polyethylene glycol and polypropylene glycol; polyols; alkanolamines; and glycerol esters. Use is also made of a substance having in its molecule a hydrophilic group such as --COOH, --OH, --NH.sub.2 or --CH.sub.2 --CH.sub.2 O--, which shows good wettability with respect to water or a substitute therefor.
However, problems with the method disclosed in Japanese patent Laid-Open No. 57-501426 are that the water absorptivity of the alginate impression material is so increased that its depolymerization due to the presence of water or the reaction of the alkaline component with the acidic component is promoted, resulting in noticeable deterioration of quality and remarkable reductions in storage and service life.
Nowhere in this specification is referred to the surface accuracy of a gypsum model. The coating agents disclosed therein have a demerit of interfering with or delaying the solidification of gypsum, so that the surface of a gypsum model obtained by pouring gypsum slurry into an impression's negative cast is roughened with a decrease in its accuracy.
Referring next to the technique disclosed in Japanese Patent Laid-Open No. 59-225104, it has for its object to provide a nonionic surface active agent and a hydrophobic liquid having a vapor pressure of 3.15 mmHg or below at 20.degree. C.
This technique is characterized in that scattering of dust is reduced with improvements in storage stability by said hydrophobic liquid component comprising combinations of hydrocarbons, fatty acids, alcohols, oils, silicone and so on, but fails to smooth the surface of a gypsum model and thereby improve the accuracy thereof.
The nonionic surfactant does not interfere with the gelling reaction of the alginate impression material of the solidification of gypsum, but is found to be ineffective for smoothing the surface of a gypsum model.
The technique disclosed in Japanese Patent Laid-Open No. 60-105607 issued under the name of the same inventors as in Japanese Patent Laid-Open No. 59-225104 is characterized in that scattering of dust is reduced with improvements in storage stability, the surface smoothness of gypsum models and impression accuracy by using a composition comprising a combination of at least one of a liquid hydrocarbon showing hydrophobic properties and having a vapor pressure of 3.15 mmHg or less at 20.degree. C. and a silicone oil with polyvinyl pyrrolidone, but does not aim at improving dimensional stability and accuracy.
Polyvinyl pyrrolidone is effective for smoothing the surface of gypsum models and impression accuracy, and the hydrophobic hydrocarbon and silicone oil excel in terms of storage stability and the prevention of scattering of powders. However, they are all so lacking in water retention that dimensional stability of the alginate impression material is reduced.
Main object of the present invention is, therefore, to provide an alginate impression material which reduced dusting, excels in storage stability, imparts improved surface smoothness and accuracy to gypsum models obtained by pouring gypsum slurry into negative casts of impressions, and can give gypsum models of accuracy increased by improvements in dimensional stability. As a result of intensive and extensive studies made to this end, the present inventors have found that the above object is achievable by incorporating lanolin with or without at least one lanolin derivative into alginate impression material compositions.
According to the present invention accomplished with the water entraining properties of lanolin in mind, it is possible to reduce scattering of powdery alginate impression materials, improve their storage stability, smooth the surface of gypsum models with increased impression accuracy and improve dimensional accuracy due to improvement sin dimensional stability.